The present invention relates generally to an active image sensor and particularly to an active image sensor with an amorphous silicon sensing layer and a semiconductor readout layer.
Many modern imaging systems such as digital cameras, camcorders, scanners, copiers, and the like require an image sensor capable of capturing an image with high resolution and color fidelity. A majority of the imaging systems use a solid-state image sensor due to constraints on system size, weight, power consumption, heat dissipation, and durability. The ability of the imaging system to capture images with high resolution and color fidelity depends to a large extent on the image sensor used. Image sensor resolution is typically measured in number of pixels contained in the image sensor. The resolution of a solid-state sensor can be increased by shrinking the size of the pixels so that more pixels fit within a predetermined area. An image incident on an image sensor having more pixels per unit of area can be captured with greater resolution than an image sensor having fewer pixels per unit of area. Pixel geometry and symmetry between pixels can also affect the number of pixels per unit of area. Additionally, the symmetry between pixels can affect color aliasing. Other factors, such as electrical noise generated by the pixels can affect image quality. Additionally, for color images, the color fidelity of the image can be affected by a predetermined color pattern assigned to the pixels in the image sensor.
Desirable attributes of an image sensor include low electrical noise, flexibility in color pattern, optimized pixel density and geometry for increased resolution, symmetry between adjacent pixels, and maximized pixel fill-factor. Low electrical noise prevents an image signal generated by the pixel from being corrupted by noise; furthermore, noise and leakage current from an individual pixel can affect the image signal from adjacent pixels. Additionally, pixel electrical noise can be affected by pixel geometry and symmetry between pixels. Circuitry or algorithms to filter out the electrical noise can be minimized or eliminated by reducing the noise generated by each pixel. A flexible color pattern allows the image sensor to be tailored to specific applications such as a RGB color pattern for photography or video, or a CMY color pattern for printing or scanning, for example.
Pixels having a rectangular geometry can generate additional noise due to abrupt rectangle edges which create leakage current in an active device such as a photodiode, for example. A pixel geometry that eliminates noise due to abrupt edges is desirable. Further, it is desirable to maximize the active area of the pixel devoted to capturing the image so that the ratio of the active area to the pixel area approaches a fill-factor of 100%. By removing components, such as transistors and signal routing lines, from the pixel, the area that was occupied by the components can be used to maximize the active area of the pixel thereby increasing the fill-factor.
Previous image sensor designs include photo-gate active pixel sensors, bulk silicon photodiode pixel sensors with three transistors, pulsed bipolar CMOS active pixel sensors, and rectangular CMOS pixel sensors.
The photo-gate active pixel sensor utilizes four transistors inside of a CMOS photo-gate pixel for low noise operation and for electronic shuttering. However, this number of transistors results in a pixel area that is not competitive with state-of-the-art CCD image sensors that have pixel areas approaching 5.times.5 .mu.m.sup.2. The number of transistors in the photo-gate sensor results in a lower fill-factor when compared to the state-of-the-art CCD image sensor.
The pulsed bipolar CMOS active pixel sensor employs a vertical bipolar device in a CMOS process. The use of bipolar devices results in a smaller pixel area due to fewer transistors in the pixel; however, disadvantages of this design include image lag and the need to amplify an output signal from the pixel with a high .beta. gain bipolar transistor. The high .beta. gain bipolar transistor exhibits signal degradation under low current conditions and limits scalability of the pixel due to an integrating amplification stage in a readout path of the pixel. Further, an extra emitter terminal is required to prevent an overflow condition. The extra emitter terminal creates an unsymmetrical pixel layout in the CMOS process. In the pulsed bipolar CMOS active pixel sensor, pixel area is not minimized because CMOS design rules require adjacent pixels to be spaced apart to comply with design rules for N-well spacing.
The bulk silicon photodiode pixel sensor is not competitive with the state-of-the-art CCD image sensor because the photodiode and the transistors are integral to the same silicon layer and the area occupied by the transistors reduces the active area available for the photodiode resulting in a fill-factor of about 30% using 1.2 .mu.m CMOS process technology.
Rectangular CMOS active pixel sensor designs utilize a rectangular pixel geometry with the pixels laid out in a rectangular grid. It is clearly understood that the rectangular grid does not result in an optimized pixel density. Bayer's color pattern was developed as the color filter array (CFA) pattern for the rectangular grid. In the Bayer CFA pattern, color density for Red, Green, and Blue sensors is in the ratio of 1:2:1. Therefore, there are two Green pixels for each Red pixel and Blue pixel in the array. In general, the color density for the Bayer CFA pattern is not the best choice because it undersamples two of the sensor colors (Red and Blue) and the color pattern kernel is not symmetric. In the Bayer CFA pattern, linear horizontal, linear vertical, and linear diagonal image features never pass through more than two different color sensor locations because the Red and the Blue sensor locations are never adjacent to sensors of the same color. The rectangular pixel geometry is not amenable to flexible color patterning.
In FIG. 1, there is provided for purposes of illustration a prior art rectangular CMOS active pixel sensor designated generally as 100. The sensor 100 has a plurality of subpixels 101. Four of the subpixels 101 define a pixel 103, shown in heavy outline. The pixel 103 has two Green subpixels 105, one Red subpixel 107, and one Blue subpixel 109. This arrangement of the color pattern for the subpixels 101 in the pixel 103 corresponds to the Bayer CFA pattern wherein the color density of the Red subpixel 107, the Green subpixels 105, and the Blue subpixel 109 is in the ratio of 1:2:1, wherein the pixel 103 has two Green subpixels 105 for one Red subpixel 107 and one Blue subpixel 109.
FIG. 2 illustrates lineal image feature traversals across prior art sensor 100. In a first horizontal traverse as shown by an arrow 133, the image feature traverses the Green subpixels 105 and the Red subpixels 107. In a second horizontal traverse as shown by an arrow 122, the image feature traverses the Blue subpixels 109 and the Green subpixels 105. The first horizontal traverse 133 never crosses the Blue subpixels 109. Similarly, the second horizontal traverse 122 never crosses the Red subpixels 107. Because the Bayer CFA color pattern kernel is not symmetric the first horizontal traverse 133 and the second horizontal traverse 122 never make a sequential crossing of the Red subpixels 107, the Green subpixels 105, and the Blue subpixels 109 in a single traverse of the sensor 100. The same is true for a first vertical traverse as shown by an arrow 135 where only the Green subpixels 105 and the Red subpixels 107 are traversed. In a second vertical traverse as shown by an arrow 124, only the Green subpixels 105 and the Blue subpixels 109 are traversed. Additionally, in a first diagonal traverse as shown by an arrow 137, only all the Green subpixels 105 are traversed. In a second diagonal traverse as shown by an arrow 126 only the Red subpixels 107 and the Blue subpixels 109 are traversed. Because the Bayer CFA pattern lacks color symmetry between adjacent subpixels 101 a sequential traverse of the Red subpixels 107, the Green subpixels 105, and the Blue subpixels 109 consistent with the RGB color pattern is not possible for vertical, horizontal, and diagonal image features. CMOS photo-gate and photodiode implementations of an active pixel sensor exhibit crosstalk between pixels of different color due to electron leakage from an electron collection node. Additionally, crosstalk is exacerbated by any non-uniformity in distance between pixels. Such non-uniformity can result in a non-uniform crosstalk pattern especially for different color channels. Due to the rectangular geometry and the Bayer CFA pattern, a pixel is surrounded by a group of eight pixels. The pixels in the group that are horizontally or vertically adjacent to the surrounded pixel are closer to the surrounded pixel than pixels that are diagonally adjacent to the surrounded pixel resulting in a variation in distance between the surrounded pixel and the other pixels in the group. A non-uniform crosstalk pattern is created by the variation in distance.
In FIG. 3, for purposes of illustration, the relative distance between adjacent subpixels 101 of prior art sensor 100 is shown by arrows 139. Red subpixel 117 is surrounded by four Green subpixels 125 and four Blue subpixels 129. The Blue subpixels 129 are diagonally adjacent to the Red subpixel 117 and are at a greater distance from the Red subpixel 117 than the Green subpixels 125 that are abutted to the Red subpixel 117. Similarly, Blue subpixel 119 is surrounded by four Green subpixels 125 and four Red subpixels 127 with the Red subpixels 127 positioned diagonal to and at a greater distance from the Blue subpixel 119 than the Green subpixels 125. The Green subpixels 125 are abutted to the Blue subpixel 119 and are therefore closer in distance to the Blue subpixel 119 than the Red subpixels 127. Green subpixel 115 is surrounded by two Red subpixels 127, two Blue subpixels 129, and four Green subpixels 125. The two Red subpixels 127 and the two Blue subpixels 129 are abutted to the Green subpixel 115, whereas the four Green subpixels 125 are positioned diagonal to the Green subpixel 115. Because of the oversampling of Green by a factor of two to one over Red and Blue in the Bayer CFA pattern, the Green subpixel 115 is surrounded by four Green subpixels 125. For each instance of a surrounded subpixel the extra distance between the surrounded pixel and diagonally adjacent pixels can result in a non-uniform cross talk pattern. For the Green subpixel 115, the non-uniform cross talk pattern can be exacerbated because the Green subpixel 115 is not surrounded by an equal number of subpixels having the same color. The two Blue subpixels 129 and the two Red subpixels 127 may not have cross talk patterns that cancel, resulting in additional cross talk due to color imbalance that is additive with the cross talk due to the extra distance of the four Green subpixels 125 that are diagonal to the Green subpixel 115.
From the foregoing it will be apparent that there is a need for a high resolution, low noise, flexible color pattern, high fill-factor, optimized pixel geometry, high density, and low crosstalk active image sensor.